This invention relates to a method and device for separating water vapor from air and particularly to a method and device for sampling air, for example, in the field of industrial hygiene monitoring.
Air sampling is an important function in the field of industrial hygiene. In this area, monitoring contaminant vapors in the air under actual working conditions involves collecting samples of various constituents or components in air, for example, hydrocarbons or other organics. It is known to collect such samples by drawing an air sample through a tube containing chemically adsorbent or absorbent material, for example, silica gel or charcoal, whereby the various constituents in air are collected on the adsorbent or absorbent material. Thereafter the adsorbent or absorbent material is removed from the tube. The collected constituents are desorbed, for example, with a suitable solvent and subsequently analyzed by well-known analytical methods such as liquid or gas chromatography.
Typically, the tube, also called a sorbent or sample tube, consists of a glass tube with an adsorbent or absorbent material inside the tube in a front and back section separated by a porous foam section. The front section may contain, for example, about 800 milligrams of charcoal and the back section may contain, for example, about 200 milligrams of charcoal.
When a chemical of interest passes through the front section without completely sorbing on the front section, this is known as "breakthrough". The back section serves as back-up material for collecting some of the chemical not collected in the front section. At high chemical concentrations or long sampling periods, certain chemicals usually breakthrough both sections. Breakthrough is enhanced at high humidity conditions, particularly for chemicals with poor sorption properties, for example, methyl chloride.
When significant breakthrough occurs, re-sampling is necessary to obtain an accurate analysis of the air sample and thus, much time, energy and materials are wasted.
To avoid early breakthrough, consecutive samples of certain chemicals are taken for short periods of time throughout an eight hour working day to more accurately monitor the atmosphere. Thus, an extended length of time for sampling air with a sorbent tube is not possible.
Therefore, it is desired to provide a device for separating water vapor from an air sample prior to collecting certain chemicals in air on a sorbent tube to eliminate the breakthrough problem associated with collection of the various chemicals under high humidity conditions.
It is also desired to provide a device for sampling air for a longer period of time without the occurrence of breakthrough.